无法理解EclipseLink警告 [英] Can't understand EclipseLink warning

问题描述

我正在使用EclipseLink 2.3.1使用JPA 2对自引用表进行建模. 创建EntityManager时，我从EclipseLink收到奇怪的警告.

I'm using EclipseLink 2.3.1 to model self referencing table with JPA 2. I get weird warning from EclipseLink when I create the EntityManager.

[EL Warning]: 2011-11-27 14:28:00.91--ServerSession(8573456)--Reverting the lazy setting on the OneToOne or ManyToOne attribute [redirectID] for the entity class [class lp.db.model.Site] since weaving was not enabled or did not occur.

我找不到有关此警告的任何文档，并且不确定其含义. 我还想知道如何解决导致此警告出现的问题...

I couldn't find any documentation about this warning, and I'm not sure what it means. I also want to know how to solve the problem that causes this warning to appear...

我是JPA的新手，所以这可能很愚蠢. 我的程序真的很简单.这是实体定义:

I'm new to JPA so it might be a silly thing. My program is really simple. Here is the entity definition:

@Entity
@Table(name="site") 
public class Site implements Serializable {

private static final long serialVersionUID = 1L;

    @Id
    @Column(name="site_id")
    public String siteID;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="redirect_id", referencedColumnName="site_id")
    public Site redirectID;

    @Column(name="name")
    public String name;
}

这是persistence.xml:

Here is the persistence.xml:

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
    <persistence-unit name="lpdb2" transaction-type="RESOURCE_LOCAL">
        <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
        <class>lp.db.model.Site</class>
        <properties>
            <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
            <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/lpdb2"/>
            <property name="javax.persistence.jdbc.user" value="blabla"/>
        </properties>
    </persistence-unit>
</persistence>

导致此警告的代码:

Persistence.createEntityManagerFactory("lpdb2").createEntityManager();

请注意，生成的EM很好，可以用于(例如)查找元素. 另外，我可以遍历实体图-我可以在数据库中找到一个实体，然后使用redirectID字段获得另一个实体.

Note that the resulting EM is fine and can be used (for example) to find elements. Also, I can traverse the graph of entities - I can find one entity in the database and then I get another entity using the redirectID field.

推荐答案

请参见 http://wiki.eclipse. org/Introduction_to_EclipseLink_Application_Development_％28ELUG％29#Using_Weaving .

要使XxxToOne关联上的延迟获取成为可能，必须修改JPA实体的字节码(这就是编织的意思).如果未修改，则只能预先获取XxxToOne关联.

For lazy fetching to be possible on XxxToOne associations, the byte-code of the JPA entities must be modified (that's what weaving means). If it's not modified, an XxxToOne association can only be eager-fetched.

热切获取意味着每次从数据库加载Site时，也会同时加载其redirectID.通过延迟获取，您可以加载网站，并且仅当您在redirectID字段上调用方法时才(延迟)加载网站的重定向.

Eager fetching means that each time you load a Site from the database, its redirectID is also loaded. With lazy fetching, you load a site, and its redirect is only loaded (lazily) when you call a method on the redirectID field.

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